1. Field of the invention
The present invention relates to video processing amplifiers and improvements therein.
2. Description of the Prior Art
Video processing amplifiers for removing unwanted elements from the video signal, and improving the picture quality are well known, such as described in "Introduction to Solid-State Television Systems," Gerald L. Hansen, Prentice-Hall, Inc., 1969, pages 117 through 143. Such video processing amplifiers vary both in relative size and complexity according to the applications for which the camera system and/or video tape editing system is designed. Broadcasters, for example, are much more concerned about the quality of their presentation than a user who employs television cameras for simple surveillance. The broadcaster, therefore, uses more elaborate means to condition the signal prior to transmission, and the video processing amplifier circuitry is usually quite elaborate. Simple camera systems, on the other hand, normally employ a relatively small number of stages to refine the signal. Moreover, the order in which the video processing functions are performed varies with the prior art camera system being utilized, although the objectives are always the same; namely the aforementioned removal of unwanted elements from the signal and the improvement in picture quality.
In such prior art video processing amplifiers, black level distortion correction and restoration is desirable. However, in such systems known to the Inventor herein, there are numerous occasions due to the conventional dark current associated with conventional vidicon tubes when it is not desirable to reference black to the back porch of horizontal blanking as would be conventionally accomplished with conventional prior art keyed clamp DC restorers; rather it would be most preferable to reference black to the blackest point in the video signal. If, in order to accomplish this, a conventional diode were utilized, such diode would be clamping to the sync pulses since that would be the blackest portion of the video signal or, similarly, if such sync pulses were not present, then the diode would be clamping to the blanking signal because that would still be the blackest point present in the video signal. This solution, however, is not satisfactory for providing automatic black restoration or automatic black correction for preferably correcting any black level distortion present in the video signal. Moreover, prior art video processing amplifiers known to the Inventor herein require complex image enhancement circuitry as well as other complex and often redundant circuitry in processing the composite video signal input so as to remove various unwanted elements from the signal and improve the picture quality. These disadvantages of the prior art are overcome by the present invention.